The present invention relates to the field of light-emitting devices, in particular, to organic-based solid-state lasers.
Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The lasing material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as, microcavity (Kozlov et al., U.S. Pat. No. 6,160,828), waveguide, ring microlasers, and distributed feedback (see also, for instance, G. Kranzelbinder et al., Rep. Prog. Phys. 63, 729 [2000]) and M. Diaz-Garcia et al., U.S. Pat. No. 5,881,083). A problem with all of these structures is that in order to achieve lasing it was necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities since this generally results in more compact and easier to modulate structures.
A main barrier to achieving electrically-pumped organic lasers is the small carrier mobility of organic material, which is typically on the order of 10xe2x88x925 cm2/(Vxe2x88x92s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (V. G. Kozlov et al., J. Appl. Phys. 84, 4096 [1998]). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales as the carrier mobility), low carrier mobilities result in orders of magnitude more charge carriers than single excitons; one of the consequences of this is that charge-induced (polaron) absorption can become a significant loss mechanism (N. Tessler et al., Appl. Phys. Lett. 74, 2764 [1999]). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of xcx9c100 W/cm2 (M. Berggren et al., Nature 389, 466 [1997]), and ignoring the above mentioned loss mechanisms, would put a lower limit on the electrically-pumped lasing threshold of 1000 A/cm2. Including these loss mechanisms would place the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, which can be supported by organic devices (N. Tessler, Adv. Mater. 19, 64 [1998]).
One way to avoid these difficulties is to use crystalline organic material instead of amorphous organic material as the lasing media. This approach was recently taken (J. H. Schon, Science 289, 599 [2000]) where a Fabry-Perot resonator was constructed using single crystal tetracene. By using crystalline tetracene larger current densities can be obtained, thicker layers can be employed, since the carrier mobilities are on the order of 2 cm2/(Vxe2x88x92s), and polaron absorption is much lower. This resulted in room temperature threshold current densities of approximately 1500 A/cm2. One of the advantages of organic-based lasers is that since the material is typically amorphous, the devices can be formed inexpensively and they can be grown on any type of substrate. The single-crystal organic-laser approach obviates both of these advantages.
A few others have suggested pumping the organic laser cavity with light-emitting diodes (LED""s), either inorganic (M. D. McGehee et al., Appl. Phys. Lett. 72, 1536 [1998]) or organic (Berggren et al., U.S. Pat. No. 5,881,089). McGehee et al. (M. D. McGehee et al., Appl. Phys. Lett. 72, 1536 [1998]) state that they needed to lower their thresholds by at least an order of magnitude to attempt laser pumping using an InGaN LED. Berggren et al. (U.S. Pat. No. 5,881,089) propose making an all organic unitary laser where one section of the device (the organic LED part) provides the incoherent radiation, while the adjacent section (the laser cavity) provides optical down conversion, gain and optical feedback. Berggren et al. state that the lasing cavity should be either a waveguide with facets, a distributed-feedback waveguide cavity, a distributed-Bragg-reflector waveguide cavity, or a photonic-lattice cavity. Berggren et al. only showed data for the organic light-emitting diode (OLED) section of the device (its current-voltage and voltage-luminance characteristics). With respect to the device""s lasing characteristics, their only comment was that it produced coherent radiation at xcx9c620 nm. Since Berggren et al. never gave any additional details with respect to the device""s lasing operation, it is difficult to determine if the device lased as a result of excitation from the OLED section of the device.
It is an object of the present invention to provide an improved arrangement for using light produced by an incoherent light-emitting device as input to a laser cavity structure for producing laser light. It has been found that a vertical laser cavity is particularly suitable for receiving incoherent light from the incoherent light-emitting device.
This object is achieved by a laser emitting apparatus, comprising:
a) an incoherent light-emitting device having a light-emitting layer and a means for applying an electric field across the light-emitting layer to produce light which is transmitted out of the incoherent light-emitting device;
b) a vertical laser cavity structure disposed to receive light transmitted from the incoherent light-emitting device, such structure including:
(i) first means for receiving light from the incoherent light-emitting device and being mainly transmissive or reflective over predetermine ranges of wavelengths;
ii) an organic active layer for receiving light from the incoherent light-emitting device and from the first light-receiving means and for producing laser light; and
iii) second means for reflecting light from the organic active layer back into the organic active layer, wherein a combination of the two means transmits the laser light.
It is an advantage of the present invention to use a vertical cavity design incorporating high reflectance dielectric multilayer mirrors for both the top and bottom reflectors and to have the active material composed of small-molecular weight organic material. As a result the laser cavity has a very low threshold. This is a consequence of: 1) the small active volume; 2) the usage of very low-loss, high-reflectivity dielectric mirrors; 3) the lasing medium being composed of small-molecular weight organic materials which can be deposited very uniformly over the bottom dielectric stack; and 4) the lasing medium being composed of a host organic material (absorbs the incoherent radiation) and a small volume percentage dopant organic material (emits the laser light) which results in a high quantum efficiency and low scattering/absorption loss. It was also found, quite unexpectedly, that the threshold power density dropped by orders of magnitude as a result of significantly increasing the cross-sectional area and pulse width (on the order of microseconds) of the pump light beam. The consequence of the very low threshold for the vertical laser cavity is that it is unnecessary to use high power density devices (focused laser light) in order to cause the cavity to lase. As a result, low power density devices, such as unfocused OLED radiation, are sufficient light sources to be used for pumping the laser cavities. Combining an organic-based laser cavity with an OLED pump source results in an inexpensive and versatile laser source whose light output can be tuned over a large wavelength range.